Dc circuit breaker

ABSTRACT

Disclosed is a DC circuit breaker capable of interrupting fault currents flowing in both forward and backward directions. The DC circuit breaker includes: a first mechanical switch for interrupting a current in a DC transmission line; a first diode connected in parallel with the first mechanical switch; a second mechanical switch, connected in series with the first mechanical switch, for interrupting a current in the DC transmission line; a second diode connected in parallel with the second mechanical switch; an LC circuit connected in parallel with the first and second mechanical switches and including a capacitor and a reactor connected in series to induce LC resonance; a first unidirectional switching device connected in parallel with the LC circuit and switching a current to induce LC resonance; and a bidirectional switching device connected in series with the LC circuit and switching currents flowing in both forward and backward directions.

TECHNICAL FIELD

The present invention relates to a direct current (DC) circuit breakerand, more particularly, to a DC circuit breaker capable of interruptingfault currents flowing through a DC transmission line in both directionswith respect to a mechanical switch when a fault occurs on the DCtransmission line.

BACKGROUND ART

A direct current circuit breaker (DC circuit breaker) is used tointerrupt a fault current when a fault occurs on a direct currenttransmission line used as a high voltage transmission line. A directcurrent transmission line for a high voltage is used as a transmissionline for a high voltage of 50 kV or higher of a high voltage directcurrent (HVDC) transmission system or a transmission line for a mediumvoltage of 50 kV or lower of a medium voltage direct current (MVDC)distribution system.

A DC circuit breaker is provided with a relatively inexpensivemechanical switch to interrupt a fault current when a fault occurs on aDC transmission line. The mechanical switch is opened to interrupt afault current, thereby preventing a faulty system from influencing anormal system when a fault occurs in a high voltage direct current(HVDC) transmission system or a medium voltage direct current (MVDC)distribution system.

However, when the mechanical switch is opened to interrupt a faultcurrent, an arc is generated in the mechanical switch due to a highvoltage applied thereto. Once an arc is generated, a fault currentcontinuously flows through the arc. Therefore, there is a problem thatthe fault current is not reliably interrupted.

In order to solve this problem, Korean Patent No. 1183508 and JapanesePatent Application Publication No. 1984-068128 propose DC circuitbreakers that extinguish an arc generated in a mechanical switch using aresonance current. In the conventional DC circuit breakers, in order tointerrupt a fault current, an arc generated when the mechanical switchis switched off due to a fault is extinguished using a technology inwhich the arc is extinguished by making a zero current which is formedby superposing a resonance current (reverse current) on a fault currentflowing through the arc in the mechanical switch.

However, the conventional DC circuit breakers have a problem thatinterruption speed is slow because the resonance current is generatedthrough multiple resonance cycles. In addition, since the conventionalcircuit breaker consists of a single resonance circuit and a singlemechanical switch connected together, it is possible to interrupt only afault current flowing in one direction with respect to the mechanicalswitch.

In addition, the conventional DC circuit breakers have a problem thatthey have a high transient recovery voltage (TRV), which is a voltageapplied between two contacts of a circuit breaker after a fault currentis interrupted. Specifically, as to a high voltage DC circuit breaker,after a fault current is interrupted, a high TRV is applied between twocontacts thereof under a certain circuit condition of a system. In thiscase, for reliable interruption of a fault current, the DC circuitbreaker needs to withstand the TRV so that no current can flow betweenthe contacts thereof. However, since conventional DC circuit breakershave a high TRV, a dielectric breakdown between the contacts is likelyto occur in the DC circuit breakers. In order to solve this problem,additional measures, such as injection of gas, are required.

DISCLOSURE Technical Problem

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the related art, and an object of thepresent invention is to provide a DC circuit breaker that can reliablyinterrupt a fault current in a mechanical switch even without applying aresonance current to the mechanical switch when opening the mechanicalswitch in the DC circuit breaker.

In addition, another object of the present invention is to provide a DCcircuit breaker capable of interrupting fault currents flowing in bothforward and backward directions when a fault occurs at one side or aremaining side thereof.

In addition, a further object of the present invention is to provide aDC circuit breaker capable of lowering a transient recovery voltage(TRV) applied between contacts thereof immediately after the a faultcurrent is interrupted.

Technical Solution

In order to accomplish the above object, the present invention providesa DC circuit breaker including: a first mechanical switch installed on aDC transmission line and being opened to interrupt a current in the DCtransmission line when a fault occurs at one side thereof on the DCtransmission line; a first diode connected in parallel with the firstmechanical switch; a second mechanical switch connected in series withthe first mechanical switch and being opened to interrupt a current inthe DC transmission line when a fault occurs at the remaining sidethereof on the DC transmission line; a second diode connected inparallel with the second mechanical switch; an LC circuit connected inparallel with the first and second mechanical switches and including acapacitor and a reactor connected in series with each other to induce LCresonance; a first unidirectional switching device, connected inparallel with the LC circuit, for switching a current flowing in onedirection, to induce LC resonance; and a bidirectional switching device,connected in series with the LC circuit, for switching currents flowingin both forward and backward directions.

In the present invention, the bidirectional switching device includes apair of power semiconductor switches that are turn-on/turn-offcontrollable, are connected in parallel, and are arranged to be counterto each other, and the first unidirectional switching device includes apower semiconductor switch that is turn-on/turn-off controllable.

In the present invention, when a fault occurs at the one side of the DCcircuit breaker on the DC transmission line, in a state in which thefirst mechanical switch is open and the bidirectional switching deviceis in an OFF state, the first unidirectional switching device is turnedon such that the capacitor is charged to a voltage −Vc through LCresonance between the capacitor and the reactor of the LC circuit, andsubsequently the first unidirectional switching device is turned off andthe first power semiconductor switch of the bidirectional switchingdevice is turned on such that a current is supplied through a closedcircuit of the second mechanical switch and the first diode due to thevoltage −Vc charged in the capacitor, thereby preventing a current frombeing supplied to the first mechanical switch.

In the present invention, when a fault occurs at the one side of the DCcircuit breaker on the DC transmission line, opening the firstmechanical switch and changing the capacitor to the voltage −Vc throughthe first unidirectional switching device that is turned on aresimultaneously performed, or sequentially performed in this order or ina reverse order.

In the present invention, as a current is supplied to the first diode, atransient recovery voltage (TRV) generated in the first mechanicalswitch is lowered by a reverse voltage that is applied between terminalsof the first mechanical switch due to the current supply to the firstdiode.

In the present invention, when a fault occurs at the remaining side ofthe DC circuit breaker on the DC transmission line, the secondmechanical switch is opened and the second power semiconductor switch ofthe bidirectional switching device is turned on such that a current issupplied through a closed circuit of the first mechanical switch and thesecond diode due to a voltage +Vc stored in the capacitor, therebypreventing a current from being supplied to the second mechanicalswitch.

In the present invention, when a fault occurs at the remaining side ofthe DC circuit breaker on the DC transmission line, opening the secondmechanical switch and supplying the current through the closed circuitof the first mechanical switch 110 and the second diode using thevoltage +Vc stored in the capacitor are simultaneously performed orsequentially performed in this order or a reverse order.

In the present invention, as a current is supplied to the second diode,a TRV generated in the second mechanical switch is lowered by a reversevoltage applied between terminals of the second mechanical switch.

In the present invention, the DC circuit breaker further includes asecond unidirectional switching device connected in parallel with thefirst unidirectional switching device and switching a current flowing ina reverse direction with respect to the current switched by the firstunidirectional switching device to induce LC resonance in the LCcircuit.

In the present invention, after a fault current is interrupted in thesecond mechanical switch, the second unidirectional switching device isturned on such that the capacitor is charged to a voltage −Vc throughthe LC resonance caused by the capacitor and the reactor.

In the present invention, the DC circuit breaker further includes aresistor installed between a ground and a contact point between the LCcircuit and the bidirectional switching device.

Advantageous Effects

As described above, the present invention can reliably interrupt a faultcurrent by easily and rapidly extinguishing an arc when the arc isgenerated during a switching operation of a mechanical switch providedin a DC circuit breaker

In addition, the DC circuit breaker according to the present inventioncan interrupt fault currents flowing in both forward and backwarddirections with a single circuit.

In addition, the DC circuit breaker according to the present inventioncan lower a transient recovery voltage (TRV) applied between twocontacts thereof immediately after a fault current is interrupted,thereby improving operation reliability for interruption of a faultcurrent.

DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram of a DC circuit breaker according toone embodiment of the present invention;

FIGS. 2A to 2D are schematic diagrams illustrating a fault currentinterruption process in the DC circuit breaker of the embodiment of thepresent invention when a fault occurs at one side of the DC circuitbreaker on a DC transmission line; and

FIGS. 3A to 3D are schematic diagrams illustrating a fault currentinterruption process in the DC circuit breaker when a fault occurs at aremaining side of the DC circuit breaker on the DC transmission line.

MODE FOR INVENTION

Hereinafter, a preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings. In addition,descriptions of known functions or constructions which have been deemedto unnecessarily obscure the gist of the present invention will beomitted below.

FIG. 1 is a configuration diagram of a DC circuit breaker according toone embodiment of the present invention.

With reference to FIG. 1, according to one embodiment of the presentinvention, a direct current (DC) circuit breaker 100 includes a firstmechanical switch 110 installed on a DC transmission line 10 connectinga first side (A side) and a second side (B side) to each other and asecond mechanical switch 130 connected in series with the firstmechanical switch 110 and installed on the DC transmission line 10. Thefirst and second mechanical switches 130 function to block the DCtransmission line to prevent a fault current from being supplied to afaulty system when a fault occurs at the first side (A side) or thesecond side (B side). To this end, the first mechanical switch 110 andthe second mechanical switch 130 are closed in normal condition butopened when a fault occurs. Specifically, the first mechanical switch110 is opened when a fault occurs at the first side (A side) on the DCtransmission line 10, thereby interrupting a current flowing through theDC transmission line 10, and the second mechanical switch 130 is openedwhen a fault occurs at the second side (B side) on the DC transmissionline 10, thereby interrupting a current flowing through the DCtransmission line 10. Operations of the first and second mechanicalswitches 110 are controlled by a controller (not shown).

In addition, according to the embodiment, the DC circuit breaker 100includes a first diode 120 connected in parallel with the firstmechanical switch 110 and a second diode 140 connected in parallel withthe second mechanical switch 130. As illustrated in FIG. 1, the firstand second diodes 120 and 140 are formed to operate in counterdirections. Specifically, the first diode 120 is formed such that acurrent is supplied from the A side to the B side but the second diode140 is formed such that a current is supplied from the B side to the Aside.

Since a high voltage is applied to the DC transmission line 10 accordingto the present invention, when the first and second mechanical switches110 and 130 are opened as a fault occurs at the A side or the B side, anarc is generated between two contacts of each of the first and secondmechanical switches 110 and 130. For this reason, although the first andsecond mechanical switches 110 and 130 are open, a fault current canflow through the arc, thereby continuously flowing through the DCtransmission line 10. Accordingly, according to the present invention,an additional device is used to reliably interrupt a fault current thatis likely to flow through the arc.

Specifically, the DC circuit breaker 100 according to the embodiment ofthe present invention further includes an LC circuit 150 connected inparallel with the first and second mechanical switches 110 and 130, afirst unidirectional switching device 160 that is connected in parallelwith the LC circuit 150 and which switches a current to induce LCresonance, and a bidirectional switching device 170 connected in serieswith the LC circuit 150 to switch currents flowing in both forward andbackward directions. Herein, according to another embodiment, the DCcircuit breaker 100 may further include a second unidirectionalswitching device 190 that is connected in parallel with the firstunidirectional switching device 160 and which switches a reverse currentwhich is counter to the current switched by the first unidirectionalswitching device 160 to induce LC resonance in the LC circuit 150.

The LC circuit 150 includes a capacitor 151 and a reactor 152 connectedin series. The capacitor 151 and the reactor 152 cause LC resonance inaccordance with switching operations of the first unidirectionalswitching device 160 or the second unidirectional switching device 190.The capacitor 151 can be charged in various ways. For example, thecapacitor 151 can be charged by a current flowing through the DCtransmission line or by an additional charging circuit.

The bidirectional switching device 170 includes two power semiconductorswitches G1 and G2 arranged to be counter to each other and connected inparallel with each other, thereby switching currents flowing in bothforward and backward directions. The two power semiconductor switches G1and G2 are arranged to be counter to each other. The first and secondunidirectional switching devices 160 and 190 respectively include apower semiconductor switch G3 and a power semiconductor switch G4,thereby controlling the flow of a current in a single direction.Although not illustrated in the drawings, switching operations of thepower semiconductor switches are controlled by the controller (notshown).

In the embodiment, the power semiconductor switches G1 to G4 are turn-oncontrollable devices and may be thyristors, for example. Alternatively,the power semiconductor switches G1 to G4 are turn-on/turn-offcontrollable devices and may be gate turn-off thyristors (GTO),insulated gate commutated thyristors (IGCT), or insulated gate bipolartransistors (IGBT), for example.

In addition, in the DC circuit breaker 100 according to the presentinvention, a resistor 180 is connected between a ground GND and acontact point between the LC circuit 150 and the bidirectional switchingdevice 170. The capacitor 151 of the LC circuit 150 is initially chargedto a voltage +Vc through the resistor 160. That is, in initial normalcondition, the first and second mechanical switches 110 and 130 areclosed and the capacitor 151 is charged to the voltage +Vc by thecurrent flowing through the DC transmission line. Charging the capacitor151 using the resistor 180 is only an example. According to anotherexample, the capacitor 151 may be charged in various ways. For example,the capacitor 151 can be charged using an external power source or anadditional charging circuit (not shown).

In the embodiment, a resistor 200 is connected in parallel with thefirst and second mechanical switches 110 and 130. Thus, when the firstand second mechanical switches 110 and 130 are opened to interrupt acurrent, the resistor 200 prevents an overvoltage higher than a ratedvoltage from being applied between terminals of the DC circuit breaker100. That is, when a high voltage that is equal to or higher than apredetermined reference voltage is applied between the terminals of theDC circuit breaker 100 due to a certain fault, the high voltage isconsumed by the resistor 200. The resistor 120 may be a varistor, asurge arrester, or the like.

FIGS. 2A to 2D are schematic diagrams illustrating a fault currentinterruption process when a fault occurs at one side (A side) of the DCcircuit breaker according to the embodiment of the present invention,and FIGS. 3A to 3D are schematic diagrams illustrating a fault currentinterruption process when a fault occurs at the remaining side (B side)of the DC circuit breaker according to the embodiment of the presentinvention.

First, as illustrated in FIG. 2A, in normal condition, the first andsecond mechanical switches 110 and 130 of the DC circuit breaker 100 areclosed. In addition, the unidirectional switching device 160 and thebidirectional switching device 170 are turned off. Therefore, when avoltage is applied to the DC transmission line 10, a normal currentflows through the DC transmission line 10 via the first and secondmechanical switches 110 and 130, and a current also flows through thecapacitor 151 and the reactor 152 of the LC circuit 150 and the resistor180, whereby the capacitor 151 is charged to the voltage +Vc.

When a fault occurs at one side (B side) in this state, the controller(not shown) detects the fault and opens the first mechanical switch 110to interrupt a fault current in the DC transmission line 10, asillustrated in FIG. 2B. When the first mechanical switch 110 is opened,an arc is generated between the contacts of the first mechanical switch110, resulting in a fault current (depicted in a dotted line)continuously flowing from the A side to the B side.

At this point, as illustrated in FIG. 2C, the pair of powersemiconductor switches G1 and G2 of the bidirectional switching device170 are turned off, and the power semiconductor switch G3 of the firstunidirectional switching device 160 is turned on. Thus, LC resonanceoccurs between the reactor 152 and the capacitor 151 through the powersemiconductor switch G3 of the first unidirectional switching device160, and, as a result, the capacitor 151 is charged to a voltage −Vc.

Afterwards, as illustrated in FIG. 2D, the first unidirectionalswitching device 160 is turned off, and the first power semiconductorswitch G1 of the bidirectional switching device 170 is turned on. Thus,due to the voltage −Vc charged in the capacitor 151, a current flowsalong a closed circuit of the first power semiconductor switch G1, thesecond mechanical switch 130, and the first diode 120. Therefore, thecurrent is not supplied to the first mechanical switch 110. Themagnitude of the current is determined depending on the capacity of thecapacitor 151.

In this state, as illustrated in FIG. 2D, due to the voltage −Vc chargedin the capacitor 151, a current is supplied to the first diode 120, so areverse voltage is applied between terminals of the first mechanicalswitch 110 with contacts being open. That is, since the reverse voltage,which is reverse to a transient recovery voltage (TRV) that is appliedbetween the terminals of the first mechanical switch 110 after a faultcurrent is interrupted in the first mechanical switch 110, is applied,the TRV is lowered. Namely, it is possible to reliably interrupt a faultcurrent by lowering the TRV of the first mechanical switch 110 in thisway. Thus, it is possible to solve a problem of conventional arts thatrequired an additional extinguishing device due to a high TRVattributable to the fact that it is difficult to apply a reverse voltagebetween two contacts.

Meanwhile, when the first mechanical switch 110 interrupts a current, avoltage at the A side rapidly rises to be higher than that at the Bside. The increased voltage at the A side is consumed by the resistor120 connected in parallel with the first mechanical switch 110. Inaddition, the second unidirectional switching device 190 is selectivelyturned on to induce LC resonance in the LC circuit 150, resulting in thecapacitor 151 being charged again to the voltage +Vc.

In the DC circuit breaker 100 according to the embodiment of the presentinvention, reclosing the first mechanical switch 110 is possible.Namely, when the fault at the B side is fixed, the controller (notshown) closes the first mechanical switch 110 to reclose the DCtransmission line 10. When the first mechanical switch 110 is closed toform a closed circuit, in the case in which the fault is not properlyfixed or a fault occurs again, the fault current interruption process isperformed again. The reclosing is possible because the capacitor 151 ofthe LC circuit 150 is maintained at a charged state (+Vc) after thefault current is interrupted in the first mechanical switch 110.

Next, with reference to FIGS. 3A to 3D, the case in which a fault occursat the remaining side (A side) will be described as another embodiment.First, in normal condition as illustrated in FIG. 3A, the first andsecond mechanical switches 110 and 130 of the DC circuit breaker 100 areclosed. The unidirectional switching device 160 and the bidirectionalswitching device 170 are in an OFF state. Accordingly, when a voltage isapplied to the DC transmission line 10, a normal current flows throughthe DC transmission line 10 via the first and second mechanical switches110 and 130, and a current also flows through the capacitor 151 and thereactor 152 of the LC circuit 150, and the resistor 180. Thus, thecapacitor 151 is charged to the voltage +Vc.

When a fault occurs at the remaining side (A side) in this state, thecontroller (not shown) detects the fault and opens the second mechanicalswitch 130 to interrupt a fault current flowing through the DCtransmission line 10, as illustrated in FIG. 3B. When the secondmechanical switch 130 is opened, an arc is generated between thecontacts of the second mechanical switch 130, so a fault current(depicted in a dotted line) continuously flows from the B side to the Aside through the arc.

At this point, as illustrated in FIG. 3C, the first unidirectionalswitching device 160 is turned off, and the second power semiconductorswitch G2 of the bidirectional switching device 170 is turned on.Therefore, due to the voltage +Vc charged in the capacitor 131, acurrent flows along a closed circuit including the second powersemiconductor switch G2, the second mechanical switch 110, and thesecond diode 140. That is, the current cannot be supplied to the secondmechanical switch 130 that is open.

In this case, as illustrated in FIG. 3C, due to the voltage +Vc chargedin the capacitor 131, the current flows to the second diode 120 and thusa reverse voltage is applied between terminals of the second mechanicalswitch 110 with contacts thereof being open. That is, since the reversevoltage with respect to the TRV, which is a voltage applied between thecontacts of the second mechanical switch 130 after the fault current isinterrupted, is applied, the TRV is lowered. Namely, it is possible toreliably interrupt a fault current by lowering the TRV applied to thesecond mechanical switch 130.

On the other hand, when the fault current is interrupted in the secondmechanical switch 130, the voltage at the B side rapidly rises to behigher than that at the A side. The increased voltage at the B side isconsumed by the resistor 120 connected in parallel with the first andsecond mechanical switches 110 and 130. In addition, as illustrated inFIG. 3D, the second unidirectional switching device 190 may beselectively turned on to induce LC resonance in the LC circuit 150 sothat the capacitor 151 can be recharged to the voltage −Vc.

In the DC circuit breaker 100 according to the embodiment of the presentinvention, the second mechanical switch 130 can be reclosed. That is,when a fault at the A side is fixed, the controller (not shown) closesthe second mechanical switch 130, thereby forming a closed circuit inthe DC transmission line 10 again. In the case in which a closed circuitis formed by closing the second mechanical switch 130, when the fault isnot properly fixed or a fault occurs again at the A side, theabove-described process is repeated again.

Although the present invention has been described above in connectionwith preferred embodiments, the present invention is not limited to theabove embodiments. Those skilled in the art will appreciate that variousmodifications, additions, and substitutions are possible, withoutdeparting from the scope and spirit of the present invention asdisclosed in the appended claims, and all of those modifications,additions, and substitutions also fall within the technical scope of thepresent invention. Accordingly, the substantial technical protectionscope of the present invention should be defined by the technical spiritof the appended claims.

1. A DC circuit breaker comprising: a first mechanical switch (110)installed on a DC transmission line and being opened to interrupt acurrent in the DC transmission line when a fault occurs at one sidethereof on the DC transmission line; a first diode (120) connected inparallel with the first mechanical switch (110); a second mechanicalswitch (130) connected in series with the first mechanical switch (110)and being opened to interrupt a current in the DC transmission line whena fault occurs at the remaining side thereof on the DC transmissionline; a second diode (140) connected in parallel with the secondmechanical switch (130); an LC circuit (150) connected in parallel withthe first and second mechanical switches (110 and 130) and including acapacitor (151) and a reactor (152) connected in series with each otherto induce LC resonance; a first unidirectional switching device (160),connected in parallel with the LC circuit (150), for switching a currentflowing in one direction, to induce LC resonance; and a bidirectionalswitching device (170), connected in series with the LC circuit (150),for switching currents flowing in both forward and backward directions.2. The DC circuit breaker according to claim 1, wherein thebidirectional switching device (170) includes a pair of powersemiconductor switches (G1 and G2) that are turn-on/turn-offcontrollable switches, are connected in parallel, and are arranged to becounter to each other, and wherein the first unidirectional switchingdevice (160) includes a power semiconductor switch (G3) that is aturn-on/turn-off controllable switch.
 3. The DC circuit breakeraccording to claim 2, wherein when a fault occurs at the one side of theDC circuit breaker on the DC transmission line, in a state in which thefirst mechanical switch (110) is open and the bidirectional switchingdevice (170) is in an OFF state, the first unidirectional switchingdevice (160) is turned on such that the capacitor (151) is charged to avoltage −Vc through LC resonance between the capacitor (151) and thereactor (152) of the LC circuit (150), and subsequently the firstunidirectional switching device (160) is turned off and the first powersemiconductor switch (G1) of the bidirectional switching device (170) isturned on such that a current is supplied through a closed circuit ofthe second mechanical switch (130) and the first diode (120) due to thevoltage −Vc charged in the capacitor (131), thereby preventing a currentfrom being supplied to the first mechanical switch (110).
 4. The DCcircuit breaker according to claim 3, wherein when a fault occurs at theone side of the DC circuit breaker on the DC transmission line, openingthe first mechanical switch (110) and changing the capacitor (151) tothe voltage −Vc through the first unidirectional switching device (160)that is turned on are simultaneously performed, or sequentiallyperformed in this order or in a reverse order.
 5. The DC circuit breakeraccording to claim 3, wherein as a current is supplied to the firstdiode (120), a transient recovery voltage (TRV) generated in the firstmechanical switch (110) is lowered by a reverse voltage that is appliedbetween terminals of the first mechanical switch (110) due to thecurrent supply to the first diode (120).
 6. The DC circuit breakeraccording to claim 2, wherein when a fault occurs at the remaining sideof the DC circuit breaker on the DC transmission line, the secondmechanical switch (130) is opened and the second power semiconductorswitch (G2) of the bidirectional switching device (170) is turned onsuch that a current is supplied through a closed circuit of the firstmechanical switch and the second diode (140) due to a voltage +Vc storedin the capacitor (151), thereby preventing a current from being suppliedto the second mechanical switch (130).
 7. The DC circuit breakeraccording to claim 6, wherein when a fault occurs at the remaining sideof the DC circuit breaker on the DC transmission line, opening thesecond mechanical switch (130) and supplying the current through theclosed circuit of the first mechanical switch (110) and the second diode(140) using the voltage +Vc stored in the capacitor (151) aresimultaneously performed or sequentially performed in this order or areverse order.
 8. The DC circuit breaker according to claim 6, whereinas a current is supplied to the second diode (140), a TRV generated inthe second mechanical switch (130) is lowered by a reverse voltageapplied between terminals of the second mechanical switch (130).
 9. TheDC circuit breaker according to claim 6, further comprising a secondunidirectional switching device (190) connected in parallel with thefirst unidirectional switching device (160) and switching a currentflowing in a reverse direction with respect to the current switched bythe first unidirectional switching device (160) to induce LC resonancein the LC circuit (150).
 10. The DC circuit breaker according to claim9, wherein after a fault current is interrupted in the second mechanicalswitch (130), the second unidirectional switching device (190) is turnedon such that the capacitor (150) is charged to a voltage −Vc through theLC resonance caused by the capacitor (151) and the reactor (152). 11.The DC circuit breaker according to claim 1, further comprising aresistor (160) installed between a ground and a contact point betweenthe LC circuit (150) and the bidirectional switching device (170).